This invention relates generally to semiconductor lasers and, more particularly, to semiconductor laser arrays. By way of background, a semiconductor laser is a multilayered structure composed of different types of semiconductor materials, chemically doped with impurities to give them either an excess of electrons (n type) or an excess of electron vacancies or holes (p type). The basic structure of the semiconductor laser is that of a diode, having an n type layer, a p type layer, and an undoped active layer sandwiched between them. When the diode is forward-biased in normal operation, electrons and holes combine in the region of the active layer, and light is emitted. The layers on each side of the active layer have a lower index of refraction than the active layer, and function as cladding layers to confine the light in the plane of the active layer. Various techniques are used to confine the light in a lateral direction as well, and crystal facets are located at opposite ends of the structure, to provide for repeated reflections of the light back and forth in a longitudinal direction in the structure. If the diode current is above a threshold value, lasing takes place and light is emitted from one of the facets, in the plane of the active layer.
The output power of a single laser diode is far too low for many applications, and much work has been done in the area of semiconductor laser arrays. Because of the manner in which adjacent elements of a laser array couple and interact, the resultant output beam does not always have the most desirable properties. In particular, the nature of the far-field radiation pattern from a laser array is a property of the array that has proved somewhat difficult to control with precision. A single-lobed far-field pattern is usually desired, but, because the mode of oscillation needed to produce a desirable far-field pattern is not a naturally occurring one, a singlelobed pattern is not obtained from an array unless some special design approach is followed.
In general, an array of laser emitters can oscillate in one or more of multiple possible configurations, known as array modes. In the most desirable array mode, all of the emitters oscillate in phase. This is known as the 0.degree.-phase-shift array mode, and it produces a far-field pattern in which most of the energy is concentrated in a single lobe whose width is limited, ideally, only by the diffraction of light. The least desirable array mode is obtained when adjacent laser emitters are 180.degree. out of phase. This is the 180.degree.-phase-shift array mode, and it produces two relatively widely spaced lobes in the far-field distribution pattern. Multiple additional modes exist between these two extremes, depending on the phase alignment of the separate emitters. Most laser arrays operate in two or three array modes simultaneously and produce one or more beams that are typically two or three times wider than the diffraction limit.
It may also be desired to adjust the output radiation pattern from an array to meet some other criterion. For example, the laser array application may call for some of the elements of the array to produce more intense radiation than others, perhaps with more light emanating from the center of the array. The present state of the art of laser arrays does not permit such control of the elemental outputs, except by individual electrical control.
It will be appreciated from the foregoing that there is still a significant need in the design of semiconductor laser arrays, for an optical structure that provides a broad degree of control of the output from a semiconductor laser array, including the ability to control the supermodes of oscillation, and to provide that selected elements emit more radiation than others. The present invention satisfies this need.